1. Field of the Invention
The present invention relates to the manufacturing of semiconductor components, and more particularly to the manufacturing of integrated circuits.
2. Discussion of the Related Art
The manufacturing of such components involves the use of a wafer made of a semiconductor material, usually silicon, but may be for example gallium arsenide, in which various successive masking and doping steps are carried out for providing various N-type or P-type regions.
Such processes are crucial and can vary as a function of numerous parameters. Hence, during manufacturing, checking of the physical characteristics of each N or P region formed must be made in order to detect any possible deviation of the manufacturing process.
Doping steps generally include a dopant implantation and/or prediffusion step followed by a dopant diffusion step in an oven. This diffusion step is carried out while a large number of wafers, for example 200, are simultaneously placed in the oven in boats including from 25 up to 50 wafers. In order to carry out checking, reference wafers that withstood an identical initial process are placed in the diffusion oven at the same time as the component wafers on which the areas occupied by the different layers are suitably localized. Then, the sheet resistivity (also called surface resistivity or resistivity per square) is measured according to the conventional "Four Probes" method (refer to L. B. Valdes, "Resistivity Measurement on Germanium for Transistors" Proc IRE, Feb. 1954, pp. 420-427). This resistivity is equal to the mean resistivity of the layer divided by its thickness and is expressed in ohms per square.
The conventional measurement device includes a head with four co-linear points separated by an arbitrary distance, frequently equal to 1/2.pi. cm, that is, 1.59 mm in commercialized devices, which simplifies certain mathematic expressions. A current I is injected between two points (generally, the two outermost points) and voltage V is measured between the two other points. The sheet resistivity is then equal to KV/I where K is a factor that depends on the shape of the sample. When the sample is an infinite plane surface and when the current is injected through the two outermost points, K s conventionally equal to .pi./Ln2=4.53. This case corresponds with a good approximation to the case when measurement is carried out in the middle of a wafer since a wafer having a 100-mm diameter can be considered as an infinite plane with respect to the 1.59-mm separation between the points. When measurement is carried out close to the wafer's edge, appropriate correction factors must be calculated and applied (refer to "Four Point Probe Correction Factors for Use in Measuring Large Diameter Doped Semiconductor Wafers" by David S. Perloff, J.E.C.S., Nov. 1976).
The conventional equipment includes an x-y movable table coupled to a computer and to a measurement apparatus and which provides a cartography of a reference wafer and allows for drawing curves of points having the same sheet resistivity. Such equipment indicates with a high accuracy the sheet resistivity value of a diffused layer and the uniformity of this resistivity. The equipment is used only during the start-up phase of a manufacturing process or for sophisticated checking steps.
For current manufacturing checking steps, five measurements only are carried out on a wafer, namely, one measurement in middle of the wafer and four measurements at the periphery of the four quadrants by using more or less automated apparatuses in order to obtain an automatic measurement, a display of the results, a mean difference value, and an automatic displacement of the points onto the five measurement areas, with application of suitable correction factors.
Those checking steps are used, as above indicated, with reference wafers in which diffusion is carried out on the whole wafer surface. Since, for monitoring the diffusion step of wafer batches, generally three reference wafers are used (one in the middle of the batch an one close to each extremity of this batch), this requires the provision of 300 reference wafers for manufacturing equipment in which each batch includes 200 wafers and where 100 batches per month are fabricated. This results in non-negligible costs, as the present trend is to use wafers having increasingly large diameters, commonly 100, 150 or 200 mm, which increases the cost of a wafer.
In order to reduce this cost and loss of material, it has been devised in the prior art to level the surface of a reference wafer after the diffusion step and checking steps so as to reuse it. However, this operation is relatively expensive and critical and cannot be repeated very frequently due to the fact that the thickness of a silicon wafer is usually within the range of 0.3 to 0.5 mm.